Many attempts have been made recently to develop a new technology of securing various mechanical elements to base structures by using adhesive rather than mechanical fasteners. Two general classes of adhesive have been utilized in this technology, namely pressure sensitive and heat activated adhesive. In the case of heat activated adhesive, the use of induction heating techniques were the logical choice as the energy source for activating the adhesive when at least one of the elements to be bonded had ferromagnetic characteristics. The main advantage of induction heating techniques over more customary heating methods is the ability to raise the temperature of metallic surfaces to high levels in a relatively short time. Due to the fact that heat is generated in the body to be heated and does not rely on heat transfer from the heat source to the body, the process is not associated with a general time lag characterizing other methods. Because of the short heating times involved, it becomes of utmost importance to devise techniques which would permit the exact control of the final temperature, either for the purpose of terminating the heating process at the instant when the final temperature was reached or to be able to maintain a desired final temperature at a constant level for an arbitrarily selected time period.
A special problem in this regard arises in the application where the ferromagnetic material to which the parts must be bonded is covered with a nonconductive, nonferromagnetic layer, such as for instance a layer of paint or other coating. All such coatings will have an upper temperature limit which cannot be exceeded without severely damaging the coating. At the same time, it is desirable to maintain the temperature of the coating at a temperature sufficient to activate the adhesive for a certain period of time in order to obtain satisfactory bonding. However, only the surface temperature of the coating and not the temperature at the interface between the coating and the sheet of ferromagnetic material is accessible for purposes of measurement and control. Previous methods utilized to control the final temperature of a surface incorporated what is generally referred to in control engineering as "proportionate control methods" i.e. reducing the power input to the induction heating generator as a function of the rate of temperature rise or using the two position method, particularly in connection with high power vacuum tube generators, using magnetic amplifiers or similar techniques to disengage the high voltage rectifier system supplying power to the oscillator tube. There are also several methods developed in industry where solid state generators were used and control was exercised by the use of power input to the workpiece, using pulse width modulation techniques or similar methods. In one example of prior art methods, control is exercised by changing the repetition rate of pulses having equal energy content.
The major problem associated with previous methods is the difficulty in avoiding overshooting the desired temperature levels or the complexity of circuitry to accomplish such a goal. In either event it is apparently necessary to establish control settings by rather complex experimental methods.